One of the key requirements in the reduction of the cost of ownership of silicon solar cells is the fabrication of high efficiency cell features at low cost. Standard industrial solar cell manufacture traditionally utilises a homogeneously heavily doped top surface emitter layer to separate charge carriers within a solar cell, transport carriers laterally to the front cell metal contacts and provide low resistance ohmic contact to these metal contacts. In contrast, a solar cell having a selective emitter utilises selectively heavily doped regions under the metal contacts while the surrounding top surface layer is comparatively lightly doped. This allows for low recombination in the lightly doped surface region, increasing the response of the cell to short wavelength light, in turn improving the short circuit current and open circuit voltage of the cell. Meanwhile, the heavily doped regions under the cell contacts reduce recombination at the metal-silicon interface, further increasing the open circuit voltage, whilst providing a low resistance ohmic contact between the silicon and the metal.
Furthermore, the cost of a solar cell is heavily influenced by the choice of metallisation process. Conventional screen printed solar cells dominate commercial manufacture however they require large amounts of thick-film silver paste to form the front contact which alone accounts for around a third of the cost of conversion of a silicon wafer into a solar cell. Conventional screen print cells possess homogeneous emitters and large metal contact regions, resulting in high recombination at metal-silicon interfaces, low short-wavelength response, high contact resistance and high shading losses. When used with fully metallised rear surface, these cells are thus limited in voltage to around 640 mV and in efficiency to about 18%. Furthermore, screen printing exerts pressure on a solar cell, necessitating a thicker silicon substrate and hence higher cost.
The latest screen printed cells, which utilise a selective emitter, overcome some of these issues by introducing heavy doping under the screen print contacts and lighter doping elsewhere. However these cells are still limited by design compromises which restrict improvements in performance and/or manufacturing cost.
Standard interconnection of screen print solar cells using lead-containing solders is also problematic because of the need for charge carriers to flow over long distances along conductive metal fingers before being collected in a central busbar which is then connected to the next cell in the module. This necessitates a large metal coverage area leading to increased shading loss and cost, while the busbars themselves also shade around 2% of the surface of the cell. To get around this problem, interconnection schemes utilising multiple evenly spaced thin busbars have been proposed. These multiple thin busbars connect directly to the fingers on the solar cell without requiring any busbar metallisation on the cell itself and allow shortened current flow distances. This may result in lower resistive losses being achieved even with solar cell contact fingers spaced slightly further apart than previously, and may also result in lower shading losses. However, until now module concepts using these interconnection schemes have focussed exclusively on the use of screen printed solar cells. Screen printed metallisation has limited scope to reduce metal usage or associated costs and fails to take full advantage of the benefits offered by multiple busbar interconnection schemes. This is because it is difficult with screen-printing to print metal layers much thinner than 10 microns without getting breaks in the metal lines. This is also the case with other metallisation approaches such as inkjet printing of metal inks, and even some plating techniques, where attempts to save costs by making narrower or thinner the metal lines leads to the creation of many breaks. Breaks in the metallisation generally lead to a reduction of power output for the cell.